The present invention relates to an apparatus for decomposing difficultly soluble substances and/or for the oxidation of organic wastes containing such difficultly soluble substances, in which the substances, in the form of a paste or viscous material, and with or without the wastes, are brought into contact with at least one hot acid and from which heat losses are compensated. This apparatus can be used particularly for decomposing PuO.sub.2 or for the decomposition and oxidation of combustible wastes containing plutonium.
Such wastes are composed mainly of PVC, polyethylene, neoprene and cellulose. Nuclear facilities generate large quantities of such wastes, and the plutonium content in such large quantities are not insignificant. For example, in one planned nuclear facility there will be produced, inter alia, about 1000 m.sup.3 annually of such wastes with a total content of 120 kg plutonium. A series of treatment methods have been developed, among which are wet combustion processes such as described in British Pat. No. 1,330,527, and U.S. Pat. No. 3,957,676, and copending U.S. Pat. application Ser. No. 141,700, assigned to the same assignee as the present application, for the purposes of volume reduction of the wastes.
One such wet combustion method is based on the chemical digestion of the wastes by means of concentrated sulfuric acid and nitric acid at a temperature of 250.degree. C. according to the following general reaction: EQU C.sub.m H.sub.n +(n/2)H.sub.2 SO.sub.4 +mHNO.sub.3 =mCO.sub.2 +nH.sub.2 O+(n/2)SO.sub.2 +m(N.sub.2,N.sub.2 O,NO,NO.sub.2)
The sulfuric acid carbonizes the wastes while the nitric acid oxidizes this carbonization product to carbon dioxide. The waste gas leaving the reactor also contains the acid gases NO, SO.sub.2 and possibly HCL which are processed into acids, separated and, in the case of HNO.sub.3 and H.sub.2 SO.sub.4, are returned to the process. An inorganic residue forms from the fillers in the wastes and the plutonium accumulates in this residue. Due to the slight difference in density and the continuous movement of the acid in the reactor, this residue remains suspended.
In the wet combustion process, difficultly soluble plutonium oxide is converted, after a suitable period of dwell, into relatively easily soluble plutonium sulfate. This chemical conversion constitutes an important step in the separation of plutonium.
A drawback of the prior art wet combustion methods, is the insufficient circulation of the plutonium oxide or the entire solid material in the reaction acid. A disclosure of one such system in which a wet combustion process is carried out is made in C. Allen et al., HDL-TME 78-77, Hanford Engineering Development Laboratory, October 1978, where a so-called tray reactor is used. The reactor includes two units, an acid heater and an oxidation furnace (digester tray vessel). Between the two units, a mammoth pump circulates the reaction acid. In the acid heater, the reaction acid (H.sub.2 SO.sub.4) is heated to a temperature of 250.degree. C., and this hot acid is brought into contact with the waste and the nitric acid on a tray of the oxidation furnace, causing the waste to oxidize. Gaseous products are formed by this reaction, and a residue remains which contains the plutonium. Instead of adding the nitric acid to the tray of the oxidation furnace and then adding the H.sub.2 SO.sub.4 from the acid heater, the nitric acid can be added at the bottom of the acid heater and then both the nitric acid and H.sub.2 SO.sub.4 are added to the tray.
In this type of reactor the heavy components of the residue preferably are deposited on the tray of the oxidation furnace and at the bottom of the acid heater. Increasing the temperature is not feasible because of the materials used, for example, for the mammoth pump. These materials also limit the service life of the system.